Umbilical cord blood (CB) is used increasingly to restore hematopoiesis in transplant patients lacking sibling, or unrelated donors. A major disadvantage of CB transplantation (CBT) is the low cell dose with resultant delays in neutrophil and platelet engraftment, as well as a high rate of engraftment failure. A major hypothesis of this grant is that ex vivo-expanded CB progenitors will provide more rapid hematopoietic reconstitution and less engraftment failure than unmanipulated CB. In the two clinical CB expansion trials conducted during the last RO1 funding period, CD133+ progenitors were isolated and cultured ex vivo in a liquid culture system targeting committed, or more primitive hematopoietic progenitors. In those sequential trials, trends in faster time to engraftment were seen, but significant losses of CD34+ progenitors occurred. To avoid the need for positive selection, which results in a substantial loss of CD34+ cells, we have developed an alternative approach that involves the ex vivo co-culture of CB mononuclear cells (MNC) with bone marrow-derived mesenchymal stem cells (MSC). We hypothesize that MSC, by functioning as a surrogate hematopoietic 'niche', will provide a more suitable microenvironment for the expansion of lineage-committed CB hematopoietic progenitors than afforded by current liquid suspension culture systems. As accrual to the current CB expansion trials nears completion, this competitive renewal application will focus on the clinical evaluation of CB expansion in MSC co-cultures, with the long-term goal of improving neutrophil and platelet engraftment in CBT patients. Although a low cell dose is clearly the chief limitation of CBT, a number of investigators have also reported a deficit in the homing of CB cells to the marrow. Thus, it is conceivable that even with optimal ex vivo expansion, inadequate homing may limit the rapidity of engraftment which is the goal of this proposal. The homing defect has been attributed to low levels of fucosylation of cell surface molecules responsible for binding to P- and/or E-selectins expressed by the marrow microvasculature. This interaction is a key component of the recruitment of hematopoietic progenitors to the marrow. We hypothesize that increasing the level of CB cell surface fucosylation will improve interactions with selectins, thereby improving homing and engraftment. Aim 3 will evaluate the modification of unmanipulated and expanded CB progenitor cells with a fucosyltransferase, as a means to facilitate their recruitment to the marrow.